FIG. 8 is a perspective view illustrating a prior art film carrier signal transmission line. In FIG. 8, reference numeral 1 designates a dielectric film such as polyimide. First metal films 3 each comprising a copper thin film or the like and having a predetermined width are disposed on a first or front surface of the dielectric material film 1 as signal lines. A second metal film 2 is disposed on a second or rear surface of the dielectric material film 1 as a ground plane.
This prior art film carrier signal transmission line operates as follows.
The film carrier signal transmission line as shown in FIG. 8 is used for such as connections between circuits in the same package, in which the copper thin film 2 functions as a ground plane and the copper thin films 3 respectively function as a signal line, and the electro-magnetic wave (hereinafter referred to as RF signal) is transmitted on the transmission line that is constituted by each of the copper thin film signal lines 3 and the copper thin film ground plane 2 of a predetermined line impedance in the Z direction Axes 100 indicate the X, Y, and Z directions.
The prior art film carrier signal transmission line of the microstrip line structure has the following problems. That is, when the respective signal lines are disposed much closer to each other with increasing integration density, mutual interference, i.e., crosstalk between signal lines, increases and transmission loss increases. FIG. 9 illustrates crosstalk between signal lines as shown by arrows 10.
In addition, at the mounting portion of the film carrier signal transmission line with a semiconductor integrated circuit operating up to millimeter-wave band frequencies and at the connecting portion of a plurality of such semiconductor integrated circuits, there occur reflections of RF signals due to impedance mismatching at the connecting portions between the transmission line and the circuits, unfavorably increasing the transmission loss.
Meanwhile, as a technique relating to the above-described film carrier transmission line, as disclosed in Japanese Published Patent Applications 3-14486 and 1-207937, in order to reduce the reflections of high frequency signals and to stabilize input and output impedance, there is provided a technique of embedding signal lines in an insulating film, providing ground planes at the both surfaces of the insulating film, and connecting the ground planes through via-holes. In this technique, however, the via-holes positioned between adjacent signal lines cannot contribute to a reduction in crosstalk, resulting in a large transmission loss as in the above-described prior art.